Method for scattered radiation correction in x-ray imaging, and x-ray imaging system for this purpose

ABSTRACT

A method is disclosed for scattered radiation correction in X-ray imaging, and an X-ray imaging system is disclosed for carrying out the method. In at least one embodiment of the method, measurement signals t from an X-ray detector are digitized and converted to logarithmic form, with these measurement signals t having been obtained by radiation through an examination object by the X-ray detector. Correction values which have been obtained from a series development of a logarithm 1n(1−s/t) are subtracted from the measurement signals that have been converted to logarithmic form, with this series development being terminated at the earliest after the first order, where s denotes a previously determined scattered radiation signal from radiation passed through the examination object. At least one embodiment of the method and the associated X-ray imaging system allow scattered radiation to be corrected for with increased accuracy, on the basis of measurement signals that had been converted to logarithmic form.

PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. §119 onGerman patent application number DE 10 2006 040 852.7 filed Aug. 31,2006, the entire contents of which is hereby incorporated herein byreference.

FIELD

Embodiments of the present invention generally relate to a method forscattered radiation correction in X-ray imaging, for example to one inwhich measurement signals from an X-ray detector are digitized andconverted to logarithmic form, with the measurement signals beingobtained from the X-ray detector when radiation is passed through anexamination object. The embodiments of the invention also generallyrelate to an X-ray imaging system which is designed to carry out themethod.

BACKGROUND

For X-ray imaging purposes, X-ray radiation is passed through anexamination object in at least one direction, and the intensity of theX-ray radiation striking an X-ray detector opposite the X-ray source ismeasured on a position-resolved basis. The measured intensity I isdependent on the absorption characteristic of the material through whichthe radiation is passed, and on the distance that the X-ray beam travelsover when passing through the object. The measured intensity I is inthis case exponentially dependent on the input intensity I₀, inaccordance with the absorption law, in which case: I/I₀=e^(−sμ(x)dx).For X-ray imaging purposes, the signal processing is generally carriedout by digitizing the measured signals and converting them tologarithmic form, thus making it possible to obtain the attenuationvalue distribution μ(y, z) or μ(x, y, z) of the examination object.Signals that have been converted to logarithmic form are also used forprocessing in computer tomography (CT).

Scattered radiation represents a fundamental problem in X-ray imagingand leads to a reduction in the image contrast, in particular toundesirable brightness smearing over the entire image. Scatteredradiation occurs because the primary radiation that propagates on astraight line between the X-ray source and the respective detectorelement is scattered on individual volume elements of the examinationobject, and the second radiation resulting from this then also strikesother detector elements in the X-ray detector, where it increases themeasured signal intensity. Despite the use of so-called scattered beamgrids upstream of the X-ray detector, it is impossible to avoid ascattered beam component in the measured signal. The scattered beamsignal which strikes the X-ray detectors is therefore frequentlysubtracted by calculation from the measured signal in order to obtainthe intensity of the primary radiation. The respective scattered beamsignal is in this case either estimated by computation or is determined,at least approximately, on the basis of previously carried outmeasurements, in particular using a measurement phantom. In this case,each detector element in the X-ray detector, for example atwo-dimensional X-ray detector array or a detector arrangement with oneor two rows, can also produce a different scattered beam signal, so thatthe scattered radiation correction is then carried out with a possiblydifferent value of the scattered beam signal for each detector element.

Until now, the scattered beam signal has been subtracted from themeasured and digitized detector signal before conversion of themeasurement signals to logarithmic form. However, this subtractionprocess can result in very small or even negative values which lead toproblems during the subsequent logarithmic conversion process, and alsogreatly increase the noise. In order to avoid this problem, it istherefore proposed that the scattered radiation be corrected bymultiplication. The corrected signal, the primary radiation p, is thenobtained from the measured signal t and the estimated scatteredradiation s using:

$p = {t\; {\frac{t}{t + s}.}}$

This correction by multiplication is obtained from an approximation, inwhich the correction by subtraction is approximated by a seriesdevelopment:

${P = {{t - s} = {{t\left( {1 - \frac{s}{t}} \right)} = {t\; \frac{1}{1 + \frac{s}{t} + \left( \frac{s}{t} \right)^{2} + \left( \frac{s}{t} \right)^{3} + \left( \frac{s}{t} \right)^{4} + \ldots}}}}},$

by terminating this sum formula for the geometric series after the firstorder.

SUMMARY

In at least one embodiment, the present invention specifies a method forscattered radiation correction in X-ray imaging, and/or an X-ray imagingsystem for this purpose, which avoid the problems of negative values andof increased noise, and lead to greater accuracy in the scatteredradiation correction.

In at least one embodiment of the present method for scattered radiationcorrection for X-ray imaging, the measurement signals t which areobtained from the X-ray detector when radiation is passed through anexamination object are digitized and converted to logarithmic form.Correction values which have been obtained from a series development ofthe logarithm 1n(1−s/t) are then subtracted from the measurement signalslnt that have been converted to logarithmic form, with this seriesdevelopment being terminated at the earliest after the first order. Inthis case, s is a previously determined scattered beam signal fromradiation being passed through the examination object.

In at least one embodiment of the present method, the scatteredradiation correction is therefore carried out only after the measurementsignals have been converted to logarithmic form, with a seriesdevelopment of the logarithm being used for correction. In this case,the values that have been converted to logarithmic form are correctedusing the following formula:

${{- \ln}\; p} = {{- {\ln \left( {t - s} \right)}} = {{{- \ln}\; {t\left( {1 - \frac{s}{t}} \right)}} = {{{{- \ln}\; t} - {\ln \left( {1 - \frac{s}{t}} \right)}} = {{{- \ln}\; t} + \frac{s}{t} + {\frac{1}{2}\left( \frac{s}{t} \right)^{2}} + {\frac{1}{3}\left( \frac{s}{t} \right)^{3}} + {\frac{1}{4}\left( \frac{s}{t} \right)^{4}} + \ldots}}}}$

In this case, the series development for the logarithm has been used inthe final step. If this series development is terminated after the firstorder, then even this surprisingly results in more accurate scatteredradiation correction than that in the case of the method described inthe introductory part, in which the series development is likewiseterminated after the first order. At least one embodiment of the presentmethod therefore results in better scattered radiation correction, inwhich case the correction can advantageously be carried out using valuesthat have already been converted to logarithmic form.

In principle, at least one embodiment of the proposed method producessensible results even in the case of estimated scattered radiationsignals which are greater than individual measurement signals, and thus,at least one of the problems mentioned further above do not occur.

In one preferred refinement of the method according to at least oneembodiment of the invention, before the correction of each measurementsignal t, a value is calculated for s/t, and the correction value isthen read from a look-up table, which has been calculated in advance andcontains correction values as a function of s/t. This look-up table cantherefore be used independently of the object through which radiation iscurrently being passed, since it contains only correction values as afunction of different ratios of s/t. Furthermore, this involves lesscomputation complexity during X-ray imaging, so that the recorded andcorrected images can be displayed in real time.

In order to further improve the accuracy for the proposed scatteredradiation correction, it is also possible to take account ofhigher-order terms, for example second, third or fourth-order terms, bynot terminating the specified series development until after terms ofthis order.

The respective scattered radiation signal is in this case determined inadvance in the same way as that already used until now in the prior art.This may be done on measurement using one or more phantoms or bycomputational estimation as a function of the thickness of theexamination object. In this case as well, of course, it is possible tocarry out different corrections for different detector elements in theX-ray detector being used, if the previously determined scatteredradiation components differ for individual detector elements.

The X-ray imaging system proposed for carrying out at least oneembodiment of the method has at least one X-ray source and one X-raydetector opposite the X-ray source, between which the examination areafor the examination object to be located in extends. The X-ray imagingsystem has a signal processing device for processing the measurementsignals which are supplied from the X-ray detector, to be precise fromthe individual detector elements in the X-ray detector. The signalprocessing device is designed such that it converts the measurementsignals to logarithmic form and subtracts correction values, which havebeen obtained from the series development of the logarithm ln (1−s/t),from the measurement signals that have been converted to logarithmicform, with the series development being terminated at the earliest afterthe first order. In this case, s represents a previously determinedscattered radiation signal for the X-ray detector, or the individualX-ray detector element, when radiation is passed through the examinationobject. The digitized measurement signal supplied from the X-raydetector or X-ray detector elements is t.

The signal processing device in this case preferably also has a memorywith a look-up table which contains the correction values as a functionof s/t, and is designed such that, before the correction of eachmeasurement signal t, it calculates the value for s/t and then reads theassociated correction value from the look-up table. The signalprocessing device or the correction table may, of course, in this casealso be designed such that the series development of the logarithm isterminated only after a higher order, for example after the second,third or fourth order.

In an X-ray imaging system such as this, the measurement signals aregenerally digitized by means of appropriate analog/digital convertersadjacent to the X-ray detector itself, or on the path between the X-raydetector and the signal processing device. However, it is possible forthe digitizing process also to be carried only in the signal processingdevice.

The proposed X-ray imaging system of at least one embodiment ispreferably a computer-tomography scanner or a C-arc system whichoperates in a similar manner, in which the measurement signals of eachprojection pass through the proposed scattered radiation correctionprocess in a corresponding manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed method and the associated X-ray imaging system will beexplained briefly once again in the following text using one exampleembodiment and in conjunction with the drawings, without any restrictionto the scope of protection stipulated by the patent claims. In thiscase:

FIG. 1 shows a schematic illustration of one example of the methodprocedure for carrying out the method according to an embodiment of theinvention;

FIG. 2 shows a comparison between the accuracy of correction beforelogarithmic conversion and correction according to an embodiment of theproposed method after logarithmic conversions; and

FIG. 3 shows a schematic illustration of one example embodiment of theproposed X-ray imaging system.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentinvention. As used herein, the singular forms “a”, “an”, and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“includes” and/or “including”, when used in this specification, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, term such as “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein are interpreted accordingly.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers and/or sections, it shouldbe understood that these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are used onlyto distinguish one element, component, region, layer, or section fromanother region, layer, or section. Thus, a first element, component,region, layer, or section discussed below could be termed a secondelement, component, region, layer, or section without departing from theteachings of the present invention.

In describing example embodiments illustrated in the drawings, specificterminology is employed for the sake of clarity. However, the disclosureof this patent specification is not intended to be limited to thespecific terminology so selected and it is to be understood that eachspecific element includes all technical equivalents that operate in asimilar manner.

Referencing the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, exampleembodiments of the present patent application are hereafter described.Like numbers refer to like elements throughout. As used herein, theterms “and/or” and “at least one of” include any and all combinations ofone or more of the associated listed items.

FIG. 1 shows an example of the method procedure for carrying out themethod according to an embodiment of the invention using a singlemeasurement signal t which is supplied from a detector element in theX-ray detector in a computer-tomography scanner. The measurement signalt is in this case converted to logarithmic form after having beendigitized in the signal processing device, resulting in the negativelogarithm −1nt. The ratio of a scattered radiation signal s which haspreviously been determined or estimated for that examination object tothe measurement signal t is then formed, and an associated correctionvalue for this ratio is read from a look-up table. The correction valueis then subtracted from the logarithm of the measurement signal, or isadded to the negative logarithm, in order to obtain the negativelogarithm of the primary signal p. This negative logarithm −1np is ameasure of the attenuation of the X-ray beam from the X-ray source tothe corresponding detector element on a straight-line path through theexamination object. The image data is then processed further on thebasis of these corrected primary radiation signals which have beenconverted to negative logarithmic form, that is to say either by thedirect image display of the image resulting from the radiation havingbeen passed through the object, or by image reconstruction in the caseof a computer-tomography scan.

FIG. 2 shows a comparison of the accuracy of the correction using thepresent method (FIG. 2 b) with the accuracy of the correction using theprior art, in which the correction is carried out before logarithmicconversion. For comparative purposes, each of the illustrations showsthe ideal exact correction by means of the solid lines. The other linesin FIG. 2 a show the result of correction by multiplication in theseries development up to the first order (line 1) for series developmentup to the second order (line 2), and for series development up to thefourth order (line 3).

In the same way, the various lines in FIG. 2 b show logarithmiccorrection using the proposed method with the series development beingterminated after the first order (line 4), with the series developmentbeing terminated after the second order (line 5), and with the seriesdevelopment being terminated after the fourth order (line 6). Comparisonof the two figures clearly shows that greater accuracy is achieved bylogarithmic correction, of the same order.

Finally FIG. 3 shows a computer-tomography scan, in a highly schematicform and as one example of the proposed X-ray imaging system, in whichan arrangement comprising an X-ray tube 8 and an opposite X-ray detector9 rotates about an examination area in a rotating frame 7. Theexamination object, a patient 10, is in this case located in a knownmanner on patient couch 11, which can be moved through the examinationarea. The measurement signals obtained from the X-ray detector aredigitized by an A/D converter, which is not illustrated but is adjacentto the X-ray detector, and are passed to a signal processing device 12,which is a component of an image computer 13 of the computer-tomographyscanner. The scattered radiation correction on the basis of the proposedmethod is carried out by way of an appropriately designed softwareprogramme in the signal processing device 12. The corrected signals arethen processed further in the image computer 13 for image reconstructionand image display on a monitor 14. By way of example, FIG. 3 also showsthe memory unit 15 with the look-up table.

Further, elements and/or features of different example embodiments maybe combined with each other and/or substituted for each other within thescope of this disclosure and appended claims.

Still further, any one of the above-described and other example featuresof the present invention may be embodied in the form of an apparatus,method, system, computer program and computer program product. Forexample, of the aforementioned methods may be embodied in the form of asystem or device, including, but not limited to, any of the structurefor performing the methodology illustrated in the drawings.

Even further, any of the aforementioned methods may be embodied in theform of a program. The program may be stored on a computer readablemedia and is adapted to perform any one of the aforementioned methodswhen run on a computer device (a device including a processor). Thus,the storage medium or computer readable medium, is adapted to storeinformation and is adapted to interact with a data processing facilityor computer device to perform the method of any of the above mentionedembodiments.

The storage medium may be a built-in medium installed inside a computerdevice main body or a removable medium arranged so that it can beseparated from the computer device main body. Examples of the built-inmedium include, but are not limited to, rewriteable non-volatilememories, such as ROMs and flash memories, and hard disks. Examples ofthe removable medium include, but are not limited to, optical storagemedia such as CD-ROMs and DVDs; magneto-optical storage media, such asMOs; magnetism storage media, including but not limited to floppy disks(trademark), cassette tapes, and removable hard disks; media with abuilt-in rewriteable non-volatile memory, including but not limited tomemory cards; and media with a built-in ROM, including but not limitedto ROM cassettes; etc. Furthermore, various information regarding storedimages, for example, property information, may be stored in any otherform, or it may be provided in other ways.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method for scattered radiation correction in X-ray imaging,comprising: digitizing and converting measurement signals t from anx-ray detector to logarithmic form, the measurement signals t havingbeen obtained by radiation through an examination object; subtractingcorrection values, obtained from a series development of a logarithm1n(1−s/t), from the measurement signals that have been converted tologarithmic form, the series development being terminated at theearliest after the first order, wherein s denotes a previouslydetermined scattered radiation signal from radiation passed through theexamination object.
 2. The method as claimed in claim 1, wherein, beforethe correction of each measurement signal t, a value is calculated fors/t, and the correction value is then read from a look-up table,calculated in advance and containing correction values as a function ofs/t.
 3. The method as claimed in claim 1, wherein the series developmentis terminated after the second order.
 4. The method as claimed in claim1, wherein the series development is terminated after the fourth order.5. The method as claimed in claim 1, wherein the previously determinedscattered radiation signal s does not have the same value for alldetector elements of the X-ray detector.
 6. An X-ray imaging system,comprising: at least one X-ray source; an X-ray detector opposite theX-ray source; and a signal processing device to process measurementsignals t produced by the X-ray detector when radiation is passedthrough an examination object, the signal processing device beingdesigned to convert the measurement signals t to logarithmic form andsubtract correction values from the measurement signals converted tologarithmic form, the correction values being obtained from a seriesdevelopment of a logarithm 1n(1−s/t) terminated at the earliest afterthe first order, wherein s denotes a previously determined scatteredradiation signal from radiation passed through the examination object.7. The X-ray imaging system as claimed in claim 6, wherein the signalprocessing device contains a memory with a look-up table, and isdesigned such that before the correction of each measurement signal t, avalue is calculated for s/t, and the correction value is then read froma look-up table, calculated in advance and containing correction valuesas a function of s/t.
 8. The method as claimed in claim 2, wherein theseries development is terminated after the second order.
 9. The methodas claimed in claim 2, wherein the series development is terminatedafter the fourth order.
 10. The method as claimed in claim 2, whereinthe previously determined scattered radiation signal s does not have thesame value for all detector elements of the X-ray detector.
 11. Themethod as claimed in claim 3, wherein the previously determinedscattered radiation signal s does not have the same value for alldetector elements of the X-ray detector.
 12. The method as claimed inclaim 4, wherein the previously determined scattered radiation signal sdoes not have the same value for all detector elements of the X-raydetector.
 13. The method as claimed in claim 8, wherein the previouslydetermined scattered radiation signal s does not have the same value forall detector elements of the X-ray detector.
 14. The method as claimedin claim 9, wherein the previously determined scattered radiation signals does not have the same value for all detector elements of the X-raydetector.
 15. A computer readable medium including program segments for,when executed on a computer device, causing the computer device toimplement the method of claim 1.